The present application relates to a positive-electrode material for a lithium-ion secondary battery, a positive electrode, a method of manufacturing the positive electrode, and a lithium-ion secondary battery employing the positive electrode.
With the recent increase in performance and multi-functionality of mobile instruments, an increase in capacity of a secondary battery as a power source thereof has been urgently desired. As the secondary battery that can satisfy the desire, a nonaqueous-electrolyte secondary battery using lithium cobaltate for a positive electrode, using graphite for a negative electrode, and an organic mixture solvent containing a lithium salt supporting electrolyte for an electrolyte has attracted attention.
In the nonaqueous-electrolyte secondary battery working with 4.2 V in maximum, a positive-electrode active material such as lithium cobaltate used for the positive electrode provides only about 60% capacity of its theoretical capacity for actual use. Accordingly, by further raising a charging voltage, the remaining capacity can be utilized in principle. Actually, it is known that it is possible to enhance the energy density by setting the charging upper limit voltage to 4.25 V or more (for example, see PCT Publication No. 03/019713). In order to cope with the new requirement for an increase in capacity, the high-capacity negative electrode employing silicon Si, germanium Ge, tin Sn, and the like has been actively studied in recent years.
The above-mentioned nonaqueous-electrolyte secondary battery is mainly used in mobile instruments such as notebook personal computers and mobile phones and is exposed to relatively high temperature due to heat emitted from the instruments or heat inside a moving vehicle for a long time. When the charged nonaqueous-electrolyte secondary battery is exposed to such an environment, gas might be generated from a reaction of the positive electrode and the electrolyte.
When the gas is generated and for example, when the nonaqueous-electrolyte secondary battery is housed in a sheath member formed of a laminate film, the sheath member is inflated to enhance the thickness thereof and thus is not fit to the specification of a battery housing of an electronic apparatus. The internal resistance of the battery increases due to the reaction of the positive electrode and the electrolyte, thereby not utilizing the sufficient capacity.
Such a phenomenon causes a problem in the batteries with the past operating voltage and remarkably occurs in batteries of which the upper limit voltage is set to 4.25 V or more. It is considered that this is because the potential of the positive electrode increases in comparison with the past system and thus the reactivity to the electrolyte is promoted. Similarly, such a phenomenon causes a problem in high-capacity batteries using silicon Si, germanium Ge, or tin Sn for the negative electrode. This is because the potential of the negative electrode is higher than the past graphite negative electrode. Accordingly, even when it is used with the past operating voltage, the potential of the positive electrode increases in comparison with the past system.
There is a problem with a cycle characteristic in the batteries using the negative electrodes. However, there has been suggested that an electrolyte containing fluorine in molecules is used, thereby greatly improving the cycle characteristic.
However, the fluorine-based electrolyte is decomposed by the positive electrode at the time of conservation at a high temperature, thereby promoting the generation of gas.
On the other hand, in order to solve the above-mentioned problems, inventors of the present application has suggested a method of using a positive-electrode active material of which the surface is coated with another compound or coating the surface with a compound at the time of manufacturing a positive-electrode coating slurry to form a stable surface layer and thus to suppress the reactivity to the electrolyte (JP-A-2007-3 35331).